how to make a adjustment knob for a gun scope model
I would suggest taking a look at creating gears.

Go to File>User Preferences>Add-Ons, search for the "Add Mesh: Extra Objects" Addon and enabled it by clicking the small checkbox next to it's name and saving your preferences.
If you now create a new mesh in your 3D scene in object mode, you will have more options available. To create a gear, create a new object by pressing 'Shift+A' and then select Mesh>Mechanical>Gears>Gear.

Before moving or editing the gear manually you have the option to configure it's settings in the "Add Gear" window that should have opened somewhere on the left by default. I suggest playing around with these values yourself until you get something that you think looks good and use it as a base to work on.